1. Field of Invention
The present invention is related to a memory and a boundary searching method thereof, and more specifically, to a non-volatile memory and a boundary searching method thereof with an error correction code (ECC) function for correcting data errors resulted from tail bits of the non-volatile memory when searching a boundary of a threshold voltage distribution of the non-volatile memory.
2. Description of Related Art
Non-volatile memory is popular and used in a wide range of electronic devices. The main characteristic of non-volatile memory is that its stored information is retained even after the power supply is terminated, thus making it suitable for portable electronic products, e.g. portable storage drives, digital cameras, mobile phones, mp3 players, etc. The information storage mechanism in a non-volatile memory cell is based on the quantity of electric charge trapped in a charge storage element of the non-volatile memory cell. The threshold voltage of the non-volatile memory cell is associated with the quantity of the trapped electric charge, and information stored in the memory cell could be distinguished based on the threshold voltage.
Along with the rapid development of science and technology at present, for storing a large amount of data, non-volatile memory is being developed to have larger capacity, higher speed, longer service life, and being more power-saving. Generally, non-volatile memory with a capacity of hundreds of M (megabytes) already cannot meet the demands, and memory with a capacity of several G (gigabytes) or hundreds of G (gigabytes) has appeared. However, the property of high capacity of the non-volatile memory may easily cause the range of the threshold voltage distribution of the memory to become excessively large.
Please refer to FIG. 1, which is diagram illustrating the threshold voltage distributions 10 and 12 of a non-volatile memory according to the prior art. The horizontal axis represents the threshold voltage, and the vertical axis represents the number of cells. The threshold voltage distribution 10 indicates a relationship between the threshold voltage and the number of memory cells having data “1”, and the threshold voltage distribution 12 indicates a relationship between the threshold voltage and the number of memory cells having data “0”. When reading data from the non-volatile memory, an operational word line voltage VOP is applied to the memory cells. Since the read-out currents of the memory cells would reflect the differences of the threshold voltages of the memory cells, the data stored in the memory cells could be determined according to values of the read-out currents. There are several tail bits 14 among the cells of the non-volatile memory. The threshold voltages of the tail bits are distributed around the edges of the threshold voltage distributions 10 and 12. Because of the existence of the tail bits 14, a boundary of the threshold voltage distribution 10 would be A1, and a boundary of the threshold voltage distribution 12 would be A2, as shown in FIG. 1. Therefore, a sensing window SW1 of the non-volatile memory located between the boundaries A1 and A2 is narrowed, such that it is difficult to distinguish each state of data stored in the non-volatile memory.